Light micrograph of a human testis seminiferous tubule containing neoplastic germ cells (round cells with pale empty-looking cytoplasm). There are no normal germ cells. The other cell type present is the Sertoli cell showing irregular shaped nuclei stained blue-purple with a dense nucleolus in the nuclei. Lipid inclusions stain green. This condition is known as germ cell neoplasia in situ (GCNIS) and is believed to be derived from arrested or dormant primordial germ cells or gonocytes in early foetal life. GCNIS cells can proliferate in the seminiferous tubules but not necessarily migrate into the intertubular tissue. In about 50 per cent of cases the cells may exhibit uncontrolled proliferation and malignant transformation, becoming invasive and leading to testicular germ cancer. Epoxy resin section, Toluidine blue stain. Magnification: x 200 when width printed at 10cm.